1. Field of the Invention
The present invention relates to a sheet coil type resolver in which sheet coils are employed as electrical conductors.
2. Description of the Related Art
A conventional sheet coil type resolver is disclosed which includes a one-phase excitation coil and a two-phase detection coil disposed to oppose the excitation coil with a distance therebetween wherein the one-phase excitation coil is constituted by a flat sheet coil such that each pole of X pole-pairs has an electrical angle of 180 degrees and wherein the two-phase detection coil includes an A-phase coil and a B-phase coil and is constituted by a flat sheet coil such that the two phases have a phase difference of 90 degrees from each other in terms of electrical angle (refer to, for example, Japanese Patent Applications Laid-Open Nos. H8-84449 and H8-136211).
A sheet coil type resolver disclosed in Japanese Patent Application Laid-Open No. H8-84449 includes a detection coil structured such that an A-phase coil pattern is formed on one surface of an insulation sheet and a B-phase coil pattern is formed on the other surface of the insulation sheet.
In a sheet coil type resolver disclosed in Japanese Patent Application Laid-Open No. H8-136211, an excitation coil is disposed at a rotor section, and a transformer coil (secondary winding) to supply excitation current to the excitation coil is disposed radially inside the excitation coil of the rotor section. The aforementioned two coils are disposed to face a disk-shaped rotor core. Another transformer coil (primary winding) is disposed at a stator section and located to oppose the transformer coil (secondary winding) disposed at the rotor section, and a transformer coil unit is composed by the two transformer coils arranged as described above. On the other hand, the stator section includes A-phase and B-phase detections coils which are disposed in two layers such that the A-phase detection coil is formed on one surface of an insulation sheet and the B-phase detection coil is formed on the other surface of the insulation sheet and which are located corresponding to the excitation coil of the rotor section, and a resolver coil is formed by the excitation coil and the A- and B-phase coil patterns of the detection coil.
However, when the A-phase coil pattern and the B-phase coil pattern are formed on respective different surfaces of the insulation sheet as described in Japanese Patent Applications Laid-Open Nos. H8-84449 and H8-136211, the distance defined between the A-phase coil pattern and the excitation phase differs from the distance defined between the B-phase coil pattern and the excitation phase, and therefore the output level of the A-phase coil pattern is caused to differ from the output level of the B-phase coil pattern. The difference in the output level causes deterioration in detection angle precision.
Also, such a resolver as described above has the problem described below. When a resolver has a relatively small number of axial multiple angle, for example, an axial multiple angle of 1X, each of both the A-phase and the B-phase coils is structured such that two rectangular spiral coil patterns circumferentially curved are each formed to extend halfway along the circumference wherein the coil patterns each include wirings extending circumferentially and disposed concentrically to one another and turn-back wirings extending radially and located toward the both circumferential end portions of the curved rectangular coil pattern.
When electric current is caused to flow in the coil described above and a magnetic flux is thereby formed, the magnetic flux variation, while staying substantially constant at the circumferential center portion of the coil pattern, becomes heavily concentrated at the circumferential end portions of the coil pattern. This is attributable to the fact that the wirings extending circumferentially and the wirings extending radially for turn-back are disposed contiguously at the circumferential end portions of the coil pattern, which has influence on the magnetic flux condition there. The influence increases with an increase in number of the axial multiple angle. The above phenomenon causes the output voltage distribution of the detection phase coil at each axis angle to deviate from a sine wave distribution. Specifically, it happens that since the change of detection output is substantially constant at the center portion of the phase, the waveform deviates from a sine wave distribution. If this happens, the detection angle precision is lowered.